1. Field of the invention
The present invention relates to light valve devices of a flat plate type, such as active matrix liquid crystal panels used for display apparatus of a direct viewing type or display apparatus of a projection type. More specifically, the invention relates to a semiconductor device that is incorporated as a substrate of the liquid crystal panel and that has electrodes and switching elements to directly drive the liquid crystal.
2. Description of the related background Art
The active matrix device is based on a simple principle in which each pixel is equipped with a switching element and when a particular pixel is to be selected, the corresponding switching element is made conductive and when it is not selected, the switching element is made nonconductive. The switching elements are formed on a glass substrate which forms part of the liquid crystal panel. Therefore, the technology for thinning and miniaturizing the switching elements is very important. Thin-film transistors are ordinarily used as such switching elements.
In an active matrix device, so far, thin-film transistors have been formed on the surface of a thin silicon film that is deposited on the glass substrate. Such transistors are generally of a field effect insulated gate type. A transistor of this type is constituted by a channel region formed in the thin silicon film and a gate electrode which is so formed as to cover the channel region. A predetermined voltage is applied to the gate electrode to control the conductance of the channel region and to carry out the switching operation.
The conventional insulated gate-type thin film transistors have a problem in that a leakage current flows into the channel region through the back side of the thin film even when the channel region is made nonconductive by controlling the gate voltage. That is, so-called back channel takes place impairing the proper operation of the active matrix device. Namely, in order to operate the pixels at high speeds by a line sequence system, the conductance ratio of the switching elements between the conductive state and the nonconductive state must be greater than 10.sup.6. In fact, however, the back channel makes it difficult to obtain the required switching performance.
Even if the back channel were extinguished, the channel region of the thin-film transistor exhibits an increased conductance if light from an external source falls thereon since the semiconductor device is used under the illuminated condition, causing leakage current in the drain and source in the nonconductive state. Further, the ratio of this leakage current to the leakage current when the semiconductor device is not illuminated increases with increase in the quality of the semiconductor thin film such as single crystal that forms the channel regions, causing another problem.
In view of the above-mentioned conventional problems, the object of the present invention is to provide a semiconductor device for driving light valves of a flat plate type having thin film transistors of a structure which is capable of effectively preventing the back channel and shutting off the incident light.